1. Field of the Invention
The present invention relates to a separation membrane for water treatment, the membrane being used in a water treatment process, especially a membrane distillation process.
2. Description of the Related Art
A membrane distillation process is performed in such a manner that phase changes occur on the surface of a hydrophobic polymer separation membrane and the resulting vapor passes through the surface micropores of the separation membrane and is thus condensed and separated. This process is applied to a desalting process for separating and removing a non-volatile material or a material having relatively low volatility, or to separation of an organic material having high volatility from an aqueous solution.
Thorough research into membrane distillation began in 1960 at which the concept of membrane distillation was first proposed, and has been mainly carried out in U.S.A., Europe, Japan, and Australia. Furthermore, recent attempts are being made to replace a conventional separation process, which uses evaporation or a reverse osmotic membrane, with a membrane distillation process.
An evaporation process and a reverse osmosis process, both currently useful for production of pure water or for desalination, require a large quantity of energy. In particular, a reverse osmosis process is problematic because a plurality of pretreatment steps has to be implemented before use attributed to pollution and fouling, making it difficult to control the operation of the process. Moreover, since this process operates at high pressure, a large quantity of electric energy is used as a pump power source, undesirably incurring high management cost.
On the other hand, a membrane distillation process using a porous membrane may operate at low pressure compared to ultrafiltration and reverse osmosis, and enables the separation due to a partial vapor pressure difference. Also, such a membrane distillation process may play a role in separating and removing a non-volatile material such as a salt, without entrainment and without the need for a filter or a separation membrane operating at high pressure, compared to typical distillation processes.
Also, in a membrane distillation process using a hydrophobic polymer separation membrane, a solvent or solute (a hydrophilic material) in a liquid phase does not pass through the membrane pores because the surface tension thereof is greater than that of the separation membrane, and is repelled from the surface of the separation membrane. Then, as the separating material is converted into a vapor phase at the entrances of the surface pores of the separation membrane, the resulting vapor is diffused into the pores, permeates the membrane, and is finally condensed and separated at the permeated side.
The membrane distillation process is implemented by a separation membrane module comprising a feed water side where a feed solution passes through a separation membrane and a treated water side where a separating material is condensed and separated.
When the membrane distillation process having the advantages described above is employed in a desalination (desalting) process, low utility cost and high durability of separation systems may result. Accordingly, this membrane distillation process is receiving attention as a competitive process in drinking water production around the world.
However, the membrane distillation process is disadvantageous because the separating material may pass through the separation membrane only when it is in a vapor phase, due to a vapor pressure difference between the feed water side and the treated water side, undesirably incurring low productivity compared to other water treatment processes.
Accordingly, there is required to develop technology for maximizing the yield of treated water in a membrane distillation system on a unit scale so as to increase water treatment efficiency.